In the Bayer process for the production of alumina, bauxite is digested in a caustic liquor, generally under conditions of elevated temperature and pressure. A variety of organic and inorganic impurities are invariably extracted at the same time, reacting with caustic soda to form their sodium salts. In addition, some of the organic compounds can undergo degradation, ultimately producing sodium carbonate and the sodium salts of a range of simple carboxylic acids. The formation of these impurities represents a major loss of caustic from the refinery's liquor streams. This caustic must either be replaced, or recovered in some way from the impurities.
The recovery of caustic from sodium carbonate is a commonplace activity in most alumina refineries. The causticisation of sodium carbonate is generally effected by the addition of lime, which reacts with the sodium carbonate to form calcium carbonate, thereby liberating sodium hydroxide. An improved version of this process is described in our co-pending International Application No. PCT/AU99/00757, filed on the 25 Sep. 1999 and entitled “Improved Bayer Causticisation”. The contents of PCT/AU99/00757 are incorporated herein by reference.
Of the other impurities, sodium oxalate and sodium sulphate are among the most significant. The presence of sodium oxalate in Bayer process streams is problematical owing to its very limited solubility. This creates a number of well-known problems within the alumina refinery. Sodium sulphate is much more soluble, and can accumulate to very high concentrations. This causes a different set of problems, particularly with respect to the refinery's productivity. The problems associated with this impurity in Bayer process liquors, and a process for its separation, have been described in Australian patent No. 673306.
Many prior art processes have been described for the removal of sodium oxalate and sodium sulphate from Bayer liquors. Some of these processes remove both impurities concurrently. In most cases, these processes advocate that the impurity is discarded after removal from the liquor stream. However, a small number of the above processes also provide a means for the recovery of soda from sodium oxalate. None describe a practical method for the recovery of soda from sodium sulphate, requiring that it be discarded. However, disposal of sodium sulphate is not straightforward.
Environmental considerations preclude disposal of sodium sulphate into natural water systems, and since it is highly soluble, it must be disposed in a suitably lined or otherwise isolated sanitary landfill if it is not to enter groundwater systems. In the alumina refinery, disposal of sodium sulphate to the red mud residue disposal areas results in the eventual return of most of the sodium sulphate to the process liquor stream with the recovered lake water.
Whilst it is preferable to utilise the sodium sulphate in some way, for example by conversion into useful products, options for this are extremely limited. Electrolytic cells are commercially available which convert sodium sulphate into sodium hydroxide and either sodium bisulphate or sulphuric acid. However, these are generally restricted to reasonably pure solutions in which scales are unlikely to form, because the membranes used in the cells are sensitive to fouling. Other processes have been investigated including reductive processes such as the Leblanc process, and the Peniakoff process for production of gibbsite from bauxite. These latter processes are not currently practised, as they are inefficient, costly and produce environmentally unacceptable by-products.
Thus, there is a significant need for an economic process for the processing of sodium sulphate into more useful products, and/or for the immobilisation of the sulphate anion in an environmentally acceptable, insoluble material.
Most alumina refineries practice some form of oxalate removal process. In general, these processes are based on variations of the following two procedures:    1. Sodium oxalate is permitted to coprecipitate with gibbsite in the refinery's gibbsite precipitation circuit. The co-crystallised oxalate reports to the refinery's gibbsite seed preparation facility, where it is removed by washing with water or dilute liquor. The oxalate-rich washings are then further treated to remove oxalate either by seeding and evaporation to recrystallise sodium oxalate or, by reaction with lime, as calcium oxalate.    2. Oxalate co-crystallisation is avoided by crystallising and removing sodium oxalate in a side-stream of one of the refinery's main process strearns (usually a spent liquor stream). The side stream is evaporated to increase the supersaturation of the sodium oxalate and directed to a series of oxalate crystallisers where it is seeded with recycled sodium oxalate crystals. After solid/liquid separation, the clarified and now oxalate-depleted liquor is returned to the process. A portion of the solid sodium oxalate is recycled to act as seed, while the remainder is either discarded or processed to recover soda. An example of this process is outlined in U.S. Pat. No. 3,899,571.
Most processes for the recovery of the soda values from sodium oxalate are based on reactions with lime. In some processes, the separated sodium oxalate cake is first burnt in a kiln to produce sodium carbonate, which is subsequently causticised by reaction with lime. This process is costly to operate, and the conversion to sodium carbonate is not always complete.
In other processes, a solution rich in sodium oxalate, such as the washings from the seed circuit of a refinery that practices coprecipitation of oxalate, is directly reacted with lime to form calcium oxalate. However, whilst very low oxalate concentrations can be achieved in the treated stream in this way, the efficiency of lime utilisation is very poor, due to the formation of calcium aluminates such as tricalcium aluminate (TCA), unless the stream is very low in caustic and sodium aluminate. Consequently, this process can only be applied to dilute liquors.